1. Technical Field
The invention is related to a plasma reactor for processing a semiconductor wafer using polymer precursor gases such as fluorocarbon gases, and in particular to a gas distribution plate and ceiling thereof.
2. Background Art
In plasma processing employed in the fabrication of semiconductor microelectronic integrated circuits, a semiconductor wafer is immersed in a plasma inside the chamber of a plasma reactor. The reactor may be thus employed to carry out any one of various processes on the wafer, such as chemical vapor deposition or reactive ion etching. In certain plasma etch processes carried out in such reactors, the upper most layer (the layer to be etched) may have a lower etch rate than the underlying layer (which must not be etched in most cases). This presents an especially challenging problem because there would be no etch selectivity (or an inverse selectivity) of the upper layer to the lower layer. This situation is characteristic of a plasma process for etching a dielectric layer overlying another layer such as a semiconductor layer (e.g., a silicon dioxide layer overlying a polysilicon layer) using a process gas containing an etchant such as fluorine or fluoride compounds. The problem has been solved by using a fluoride compound such as a fluorocarbon gas or a fluoro-hydrocarbon gas which, upon ionization, tends to break up into fluorine-containing etchant species and polymer precursor species. The polymer precursor species provide the requisite etch selectivity because it tends to accumulate as a hard polymer film on non-oxygen containing materials (such as the underlying polysilicon layer) but does not accumulate on oxygen-containing materials (such as the overlying silicon dioxide layer). Thus, the underlying layer is protected from the etchant by the polymer layer while the overlying layer is left exposed to the etchant, so that the process has a net etch selectivity of the overlying layer.
The problem is that the polymer accumulates on the interior reactor surfaces, including the ceiling of the chamber. Typically, the ceiling consists of a gas distribution plate with gas distribution inlets or orifices through which the process gas must be sprayed into the reactor chamber for uniform distribution. The plate must be formed of materials such as quartz which are suitable for carrying the etchant-containing process gases. Such materials do not readily lend themselves to temperature control, and therefore the center of the gas distribution plate tends to be very hot due to plasma heating while the perimeter tends to be colder. The polymer accumulates as a solid film in the colder perimeter region and cannot accumulate in the hot center region. Between these two regions is a transition region where the polymer tends to accumulate as a fine powder, which tends to flake onto the wafer and create contamination. This requires that the gas distribution plate be replaced periodically. The gas distribution plate is on the order of the diameter of the wafer (e.g., 9 inches or 14 inches) and its replacement is expensive due to the cost of the item as well as the non-productive time during which the reactor is disassembled for removal and replacement of the gas distribution plate. However, periodic removal and replacement of the gas distribution plate is not a solution to the problem, as flaking of any accumulated polymer from the gas distribution plate can occur any time up to the replacement of the plate.
One solution to this problem might be to cool the entire gas distribution plate so that the polymer deposited thereon is entirely of a hard consistency and will not flake. However, this would eventually block the gas inlets, stopping the inflow of the process gas. Another solution might be to heat the entire gas distribution plate sufficiently to prevent any polymer from accumulating thereon. However, this would expose the entire gas distribution plate to bombardment from the plasma and much faster wear.
Therefore, there is a need for a gas distribution plate which is not susceptible to accumulation of polymer or the flaking of accumulated polymer onto the wafer.
A plasma reactor embodying the invention includes a wafer support and a chamber enclosure member having an interior surface generally facing the wafer support. At least one miniature gas distribution plate for introducing a process gas into the reactor is supported on the chamber enclosure member and has an outlet surface which is a fraction of the area of the interior surface of said wafer support. A coolant system maintains the chamber enclosure member at a low temperature, and the miniature gas distribution plate is at least partially thermally insulated from the chamber enclosure member so that it is maintained at a higher temperature by plasma heating.